.\"	$OpenBSD: EVP_DigestSignInit.3,v 1.13 2024/07/21 08:36:43 tb Exp $
.\"	OpenSSL 9b86974e Aug 17 15:21:33 2015 -0400
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.Dd $Mdocdate: July 21 2024 $
.Dt EVP_DIGESTSIGNINIT 3
.Os
.Sh NAME
.Nm EVP_DigestSignInit ,
.Nm EVP_DigestSignUpdate ,
.Nm EVP_DigestSignFinal ,
.Nm EVP_DigestSign
.Nd EVP signing functions
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_DigestSignInit
.Fa "EVP_MD_CTX *ctx"
.Fa "EVP_PKEY_CTX **pctx"
.Fa "const EVP_MD *type"
.Fa "ENGINE *engine"
.Fa "EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_DigestSignUpdate
.Fa "EVP_MD_CTX *ctx"
.Fa "const void *d"
.Fa "size_t cnt"
.Fc
.Ft int
.Fo EVP_DigestSignFinal
.Fa "EVP_MD_CTX *ctx"
.Fa "unsigned char *sig"
.Fa "size_t *siglen"
.Fc
.Ft int
.Fo EVP_DigestSign
.Fa "EVP_MD_CTX *ctx"
.Fa "unsigned char *sigret"
.Fa "size_t *siglen"
.Fa "const unsigned char *tbs"
.Fa "size_t tbslen"
.Fc
.Sh DESCRIPTION
The EVP signature routines are a high-level interface to digital
signatures.
.Pp
.Fn EVP_DigestSignInit
sets up the signing context
.Fa ctx
to use the digest
.Fa type
and private key
.Fa pkey .
The
.Fa ENGINE *engine
argument is always ignored and passing
.Dv NULL
is recommended.
.Fa ctx
must be initialized with
.Xr EVP_MD_CTX_init 3
before calling this function.
If
.Fa pctx
is not
.Dv NULL ,
the
.Vt EVP_PKEY_CTX
of the signing operation will be written to
.Pf * Fa pctx :
this can be used to set alternative signing options.
Any existing value in
.Pf * Fa pctx
will be overwritten.
The
.Vt EVP_PKEY_CTX
value returned must not be freed directly by the application.
It will be freed automatically when the
.Vt EVP_MD_CTX
is freed.
.Pp
.Fn EVP_DigestSignUpdate
hashes
.Fa cnt
bytes of data at
.Fa d
into the signature context
.Fa ctx .
This function can be called several times on the same
.Fa ctx
to include additional data.
This function is currently implemented using a macro.
.Pp
.Fn EVP_DigestSignFinal
signs the data in
.Fa ctx
and places the signature in
.Fa sig .
If
.Fa sig
is
.Dv NULL ,
then the maximum size of the output buffer is written to
.Pf * Fa siglen .
If
.Fa sig
is not
.Dv NULL ,
then before the call
.Fa siglen
should contain the length of the
.Fa sig
buffer.
If the call is successful, the signature is written to
.Fa sig
and the amount of data written to
.Fa siglen .
.Pp
.Fn EVP_DigestSign
signs
.Fa tbslen
bytes of data at
.Fa tbs
and places the signature in
.Fa sigret
and its length in
.Fa siglen
in a similar way to
.Fn EVP_DigestSignFinal .
.Fn EVP_DigestSign
is a one shot operation which signs a single block of data
with one function call.
For algorithms that support streaming it is equivalent to calling
.Fn EVP_DigestSignUpdate
and
.Fn EVP_DigestSignFinal .
.\" For algorithms which do not support streaming
.\" (e.g. PureEdDSA)
.\" it is the only way to sign data.
.Pp
The EVP interface to digital signatures should almost always be
used in preference to the low-level interfaces.
This is because the code then becomes transparent to the algorithm used
and much more flexible.
.Pp
The call to
.Fn EVP_DigestSignFinal
internally finalizes a copy of the digest context.
This means that
.Fn EVP_DigestSignUpdate
and
.Fn EVP_DigestSignFinal
can be called later to digest and sign additional data.
.Pp
Since only a copy of the digest context is ever finalized, the context
must be cleaned up after use by calling
.Xr EVP_MD_CTX_free 3 ,
or a memory leak will occur.
.Pp
The use of
.Xr EVP_PKEY_size 3
with these functions is discouraged because some signature operations
may have a signature length which depends on the parameters set.
As a result,
.Xr EVP_PKEY_size 3
would have to return a value which indicates the maximum possible
signature for any set of parameters.
.Sh RETURN VALUES
.Fn EVP_DigestSignInit ,
.Fn EVP_DigestSignUpdate ,
.Fn EVP_DigestSignFinal ,
and
.Fn EVP_DigestSign
return 1 for success and 0 for failure.
.Pp
The error codes can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_DigestInit 3 ,
.Xr EVP_DigestVerifyInit 3 ,
.Xr EVP_PKEY_meth_set_signctx 3
.Sh HISTORY
.Fn EVP_DigestSignInit ,
.Fn EVP_DigestSignUpdate ,
and
.Fn EVP_DigestSignFinal
first appeared in OpenSSL 1.0.0 and have been available since
.Ox 4.9 .
.Pp
.Fn EVP_DigestSign
first appeared in OpenSSL 1.1.1 and has been available since
.Ox 7.0 .
